Data from an Air Force hurricane hunter plane indicated that maximum sustained winds in Hermine had increased to about 80 miles (130km) per hour at 8pm (0000 GMT), the Miami-based National Hurricane Center said, predicting some additional strengthening before landfall.

That would make it a Category One hurricane on the Saffir-Simpson scale.

Hermine is the fourth Atlantic hurricane of 2016 and the first to hit Florida in more than a decade.

Hermine was about 45 miles south of Apalachicola and moving about 14 miles per hour.

A hurricane warning was in effect for northern Florida and authorities predicted dangerous storm surges and flooding.

Heavy rains were already pummeling many parts of the southern state Thursday night, accompanied by intense wind gusts.

Northern Florida, southeast Georgia and the eastern Carolinas also faced tornado threats that could last into Friday night.

"Winds are already near tropical storm strength in portions of the warning area, making outside preparations difficult or dangerous," the NHC said.

"Preparations to protect life and property should be rushed to completion."

The NHC forecasted total rainfall accumulations of five to 10 inches in parts of northwest Florida and southern Georgia, with some isolated areas receiving 15 inches.

Pentagon spokesman Jeff Davis said 100 Florida National Guard personnel were activated, with 6,000 more on alert in the state and 34,000 members ready to deploy from elsewhere in the US.

President Barack Obama has asked FEMA administrator Craig Fugate to keep him updated on the situation "and to alert him if there are any significant unmet needs," said White House spokesman Josh Earnest.

"Local, state and federal officials have been working diligently to prepare for these storms and have resources on hand to respond to them as necessary," said Earnest.

Scott has declared a state of emergency in 51 counties to free up resources to brace for the storm.

"Anyone in the storm’s path must have ready the resources your families need such as water, flashlights, batteries and medicine," he said, warning that power could go out for days or even weeks.

The state of Florida has issued a mandatory evacuation notices in the counties of Dixie, Franklin, Taylor, Wakulla and Levy. — AFP